Electroplating zinc on aluminum



United States Patent Ofiice 3,079,310 Patented Feb. 26, 1963 3,079,310 ELECTROPLATING ZINC N ALUMINUM James V. Sheridan, Dearborn, Mich.

(137 E. Dominion Blvd, Columbus 14, (Ellie) No Drawing. Filed Aug. 26, 1960, Ser. No. 52,662 31 illaims. (Cl. 204-55) This invention concerns the electroplating of zinc on an aluminurn or aluminum alloy basis metal.

The art of electroplating. metals on aluminum has not met with the success attained by the art of electroplating on steel. Only one method has reached any degree of commercial acceptance-the zincating method. Zincating involves a zinc immersion from an alkaline zincate bath, in contrast to a zinc electrcplate. The immersion plate is very thin and, practically always, has to be stripped and replated at least once to secure adequate adhesion. Further, not all alloys of aluminum are successfully zincated by routine procedures and I have been presented with a variety of difierent aluminum alloys, particularly die cast alloys, which experienced zincaters had deemed impossible to plate.

Wean, in U.S. Patent No. 2,646,397, issued July 21, 1953, suggests that a small proportion of titanium in an acid zinc bath can refine the grain of zinc electroplates on steel and suggests that the same bath be used in the electroplating of zinc on aluminum. I have taken the Wean teachings through extensive laboratory and pilot studies and have noted the excellence of grain refinement. I have found, however, that the adhesion of the zinc on the aluminum, although comparable with some of the baths of the art, was not satisfactory for widespread commercial acceptance by platers handling a wide variety of alloys in a single installation. I was, however, impressed with the grain structure and unusual adhesion of the zinc electroplate on steel and desired to maintain these advantages while solving the problems of zinc electroplating on aluminum.

It is therefore an object of this invention to provide a bath composition and method for the electroplating of zinc on aluminum.

It is also an object of the invention to secure such an electroplate with good adhesion to the base metal with all alloys of aluminum in order that one bath, without modification, can be used routinely in a plating shop.

It is a further object to secure such an electroplate in a bath of relatively simple make-up, the bath being stable over long periods of usage, the bath being simple to analyse and simple to control, and the bath being comparable in cost with the everyday plating baths of industry.

I have now found that the foregoing and related ob jects can be secured in a plating bath consisting essentially of an aqueous solution of zinc sulfate in a concentration of at least about 175 grams per liter and fiuotitanate ions (TiF or fiuozirconate ions (ZrF in a concentration of about 0.6-10 grams per liter, the bath having a pH of about 0.5-1.9. The bath also requires the substantial absence of free titanium ions and the substantial absence of chloride.

Zinc sulfate znsop is used in concentrations of at least about 175 grams per liter, or at least about 70 grams per liter zinc ions. About 350500 grams zinc sulfate per liter are preferred in order to obtain maximum throwing power within economical limitations. In terms of zinc ions this preferred range is about l40200 grams per liter. There is no upper limit on the zinc ion concentration. Solutions saturated with zinc sulfate operate well but little can be gained by operating above about 500 grams per liter zinc sulfate.

An important feature of the invention lies in the unexpected adverse effect of chloride ions. In this connection substantial absence of chloride means less than about 3.5 grams per liter.

The pH of the bath is quite critical. Adhesion is lost above a pH of about 1.9 but is substantially perfect on all alloys of aluminum below about 1.9. The low end of the pH range depends on two factors, adhesion and throwing power. The bath can operate quite well at a pH as low as 0.5 but perfect adhesion can be secured at a pH only as low as about 1.0. As pH is lowered there is some sacrifice of throwing power. To obtain exceptional throwing power and exceptional adhesion, the pH range is narrowed to about 1.5-1.9, preferably about 1.7.

In speaking of adhesion I use the word perfect to mean that the zinc plate on aluminum resulting from the practice of my invention cannot be removed with any mechanical stress such as bending, twisting, hammering, etc.

Since acid zinc baths have notoriously poor throwing power it may be pointed out that the comparatively good throwing power of my bath is not only good relative to other acid zinc baths but is better than commercially used acid nickel and chromium baths.

The concentration of fiuotitana'te ion, or fluozirconate ion, should be between about 0.6 and about 10 grams per liter. For example, the commercial form of potassium fiuotitanate, K TiF -H 0, yields 62.7% of its weight as fluotitanate and about 1-15 grams per liter of this compound is needed. About 5 grams of fluotitanate ion or about 8 grams per liter of the K TiF -H o is optimum.

As indicated previously, free titanium ions are inhibitory to perfect adhesion of zinc on aluminum if present above a very small proportion. For example, the introduction of a few tenths of a gram per liter of trivalent or tetravalent titanium into a perfectly functioning bath will cause a noticeable loss of adhesion of electroplated zinc on aluminum. Concentrations of 0.5 gram per liter of these ions will cause the complete loss of adhesion of the zinc electroplated from the baths of the invention.

Thus, a major factor in the success of the invention is the form in which the titanium appears. For example, the Wean reference referred to above suggests titanous chloride. Such a compound yields a free titanium ion with a positive charge and a free chloride ion with a negative charge. I have now discovered the rather unique fact that although the use of the free titanium ion was a distinct advance in the art and was helpful for Weans purposes it is actually an inhibitor to the securing of perfect adhesion of zinc electroplates on aluminum. A com centration of about 0.2 gram per liter free titanium ions has significant inhibiting properties and a concentration of about 0.5 gram per liter eliminates the adhesion.

I have now found that if the titanium or zirconium is present in the negative ion complex, the fluotitanate ion or fluozirconate ion, I secure a new and extremely valuable result. The principal source of such ions is their alkali metal salts such as potassium fiuotitanate, potas sium fluozirconate, also called potassium titanium fluoride (K TiF potassium zirconium fluoride (K ZrF and the like.

It is most important to the operation of the bath that the fiuotitanate complex remain stable and not break down rapidly to form free titanium ions. Under normal operating conditions using carbon anodes the breakdown occurs very slowly. A reducing agent such as a zinc anode, or any addition to the bath involving metallic zinc, causes a relatively rapid breakdown of the complex and the formation of a free, lower valence titanium ion. Reoxidation back to the tetravalent form does not reform the complex but results in a free tetravalent titanium ion. Even restoration of the original concentration of complex titanium does not cure the deficiency if the free titanium ion is present in significant concentrations for the reason that the free ion acts as an inhibitor to perfect adhesion of zinc to an aluminum cathode. Thus, the invention calls for the substantial absence of free titanium ions and also calls for an operation wherein the bath does not contact a zinc or any other active metal surface. Thus, an inert anode must be used and carbon is by far the best material for the anode.

It is therefore a further object of this invention to provide for the removal of free titanium ions from the bath of the invention.

I have now found a novel method of accomplishing this objective. This involves the addition of activated carbon to the bath, correcting the pH to about 4.8 or higher, permitting the bath to stand for a period of time, and then filtering the bath. Once filtered the bath may be restored to its operating pH with, by way of example, sulfuric acid. This treatment was found necessary about once per month in a 1100 gallon bath used fairly regularly.

When free titanium ions appear as a result of the breakdown of the fluotitanate complex, they appear to form because of the reducing action at the cathode and because of the contact of the bath'with the freshly plated zinc. In this manner the tetravalent titanium tends to be reduced to lower valence. forms. The bath can take on a green cast at this point, which color is always a sign of trouble.

The valence forms of free titanium below four do not precipitate as the pH is raised, at least until a pH is reached which will precipitate substantial proportions of zinc hydroxide. However, the tetravalent form precipitates completely at a pH of 4.8. To oxidize the titanium back to a valence of 4 without going to a valence of 6 (eg with peroxide) or without introducing an impurity into the bath (e.g. with permanganate) provide extremely difficult. It was surprising indeed to discover that this oxidation proceeds spontaneously in the presence of activated carbon. It would appear that the carbon acts as a catalyst for the titanium ions and the oxygen dissolved in the bath. Thus, the combination of carbon and a pH above about 4.8 results in the removal by precipitation of the free tetravalent ions. It is important not to exceed a pH of 4.8 by very much in order to avoid precipitating undue proportions of zinc hydroxides.

About one pound of activated carbon is used for each 10 gallons of bath (about 10 grams per liter). The pH rise is best effected with potassium hydroxide and the readjustment back to about 1.7 is best effected with sulfuric acid.

An additional feature of the invention associated with the securing of uniform crystallinity of zinc deposits on aluminum and conversely the elimination of amorphous or non-crystalline areas on the plated surfaces involves a pretreatment of the bath of the invention.

, During the initial experience with the bath of the invention I observed that the bath tended to yield zinc deposits which were highly crystalline and bright over most of the plated surface, but the plate appeared spotted in a random fashion. The spots were small amorphous areas. These spots, particularly noticeable on silicon containing aluminum die castings, spoiled the appearance of the plate since they resembled somewhat a piece of metal which had been splashed with dirty water.

-These amorphous spots are not objectionable where the zinc plate is to be used for a non-decorative purpose; for example, soldering to a zinc plated aluminum piece. The amorphous spots are particularly objectionable, however, in decorative plating where the zinc plate on aluminum is to be covered with electroplates of copper, nickel and chromium. Unfortunately, the spots show through each of these successive plates and spoil the appearance of the chrome finish.

I have-found that the production of such amorphous spots is associated with the zinc sulfate used in the bath. A-wide variety of sources and grades of zinc sulfate have been used and all have been found to present this problem in varying degrees.

It is therefore another object of this invention to provide for the production of crystalline zinc electro-plated deposits on the aluminum basis metal and to prevent amorphous deposits where they have a tendency to occur.

It is a further object to provide a method for treating an acid zinc electroplating bath containing zinc sulfate in such a manner as to remove or offset the effect of substances tending to cause or enhance the production of amorphous spots.

I have now found that the foregoing and related objects can be secured by introducing barium ions into the bath by means of a small proportion of a soluble barium compound. About 2 grams per liter barium is the most effective quantity. This quantity is fairly critical since at about half that quantity or at about double that quantity, the treatment is only partially successful. Thus about l-4 grams of barium ion per liter yields a significant result. The most useful barium compound is barium chloride because of its ready solubility and because it does not alter the pH of the bath. About 1.5-6 grams per liter barium chloride should be used. However. other Water or acid soluble barium salts such as hydroxide, carbonate, halide, and the like can be used; it being most important that the ions of barium be available.

I am not entirely sure of the cause of the amorphous spots which appear among the crystals of zinc on the zinc electroplate. However, I believe that these amorphous spots are caused by colloidal matter which occurs in the zinc salts, which colloidal matter is not entirely soluble even at the relatively low pH of the bath. It is my belief that these colloidal particles, during the electroplating process, migrate to the cathodes, adhere to the plate, and cause the amorphous spots to appear.

When the bath is treated with a solubfe barium compound, the barium ions of course, precipitate out as barium sulfate, the latter being filtered out. No barium would remain in the bath in View of the high sulfate concentration. Whether the barium compound precipitates the colloid or whether the colloid is absorbed on the barium sulfate precipitate is not entirely clear. In any event, however, the cause of the amorphous spots disappears.

The invention requires that the aluminum basis metal be thoroughly cleaned prior to the plating procedure. I have found that degreasing with perehlorethylene and the like, etching with a suitable alkaline etch, and desmutting with an acid constitute a suitable sequence. The etching of the metal appears to be a necessity for the reason that the fabrication of aluminum results in the enveloping of aluminum oxide, lubricants, and other soils in recesses below the surface of the metal. It is my ex perience that these materials are not removed by simple surface cleaning. A mild hot alkaline etch, however, for about 30 seconds appears to undercut, or open up, and clean out these recesses.

As is well known alkaline cleaners cause a smut on the surface of the aluminum which smut must be removed. With relatively pure aluminum or with copper containing alloys such as 2024 the smut is best removed by dipping for a minute in 50-60 percent nitric acid at room temperature. With high silicon alloys the acid rinse may be fortified with ammonium bifiuoride to secure complete smut removal. For example, 70 grams per liter, or 10 ounces per gallon, ammonium bifiuoride added to an percent nitric acid solution at room temperatures gives excellent results.

As indicated previously zinc anodes cannot be used in the plating bath because zinc causes a breakdown of the complex titanium ion and causes free titanium ions to appear in the bath. Therefore an inert anode must be used and carbon is the best choice. The cathode is, of course, the aluminum piece to be plated.

A voltage of about 2-10 volts may be impressed across the electrodes, the higher voltages of 8-10 volts being used in large installations and where the highest throwing power is required. With fiat sheets and in smaller installations the lower voltage of the range are adequate. A wide range of current densities on the cathode; up to several hundred amperes per square foot, can be used but about 60 amperes per square foot appears to be best for most applications. Since the zinc electroplate on aluminum is usually for the purpose of soldering or for the purpose of an undercoat for other metals, a thick coating is seldom required. I have therefore used 60 amperes per square foot for 5 minutes to constitute a standard plating procedure. This produces about 0.00025 inch of plate.

Cathode eificiency is not 100 percent and some hydrogen bubbles form on the cathode. These may be prevented from sticking to the work piece, if desired, by using a small proportion of detergent in the bath-sodium dodecyl sulfate, for example.

The temperature of the plating bath is about 100- 140 F. with about 120 F. being optimum.

During the plating with an inert anode, zinc is deposited at the cathode and oxygen at the anode. There appears to be an over-all tendency for the' pH of the bath to drop during its use. I prefer to correct this drop in pH by adding a basic zinc compound such as zinc oxide, zinc hydroxide, or zinc carbonate. In this manner the pH can be raised to the point desired and some of the zinc depletion can be made up. Where losses are caused by drag'out these are made up by adding more of the original bath ingredients;

The invention applies to all alloys of aluminum. I have plated many hundreds of each of several dozen widely different metal compositions varying from pure aluminum to the relatively high copper, high silicon, and high magnesium aluminum alloys. Of special interest is the fact that the high copper and high silicon alloys, which pose the greatest problem to the zincater, have been plated routinely in the present bath.

Example I A 1100 gallon zinc electroplating bath was made up as follows: Five gallons of sulfuric acid were added to about 700 gallons of water in a storage tank in order to facilitate the solution of the zinc salt to be added. Enough of a commercially available hydrated zinc sulfate to yield 2700 pounds zinc sulfate (anhydrous basis) was added slowly with stirring until the zinc sulfate was all dissolved. 60 pounds of potassium fluotitanate monohydrate were then added. The solution was stirred vigorously and was then filtered into a plating tank where its volume was made up to 1100 gallons with water.

At this point the pH of the solution was 2.2. This was adjusted to 1.7 with sulfuric acid. The temperature of the bath was then brought to 120 F. Carbon anodes were used and the rectifier was adjusted at 8.0 volts and 60 amperes per square foot on the cathode.

A variety of aluminum alloys (28, 38, 528, 2024, 1100, 5055, 380, etc.) in a large variety of shapes were plated in the bath. A number of these were plated for 5 minutes and were then soldered together via the zinc coating with usual soldering techniques. Perfect joints were secured. A number of the Zinc plates were placed directly into a hard chrome bath where hard chrome was applied directly to the zinc plate. A notable example of this was the successful hard chrome surfacing of aluminum pistons from alloy 380. A number of zinc plates were plated with copper, nickel, and chrome in the usual manner to yield decorative chrome plates which matched in every way the comparable plating sequence on steel.

Although the adhesion of the zinc plates was perfect there were a number of amorphous spots on the zinc plate. These were removed as follows: The bath was returned to the storage tank where 35 pounds of barium chloride were added with stirring. The precipitate was allowed to settle and the bath was filtered back into the plating tank. When plating was resumed the amorphous spots had disappeared and all plates were uniformly crys talline.

The foregoing bath was used continuously for a 10 month period. During this period it was found that the complex titanium ion, TiF tended to break and yield free titanium ions, some tetravalent and some of lower va-lences. When this occurred to any significant degree some zinc hydroxide also precipitating. The bath was then filtered back into the plating tank and readjusted to plating conditions as previously specified.

Example 2 A 3 liter bath was made up as follows: 900 grams of hydrated zinc sulfate (89.9 percent anhydrous) were added to about 2200 ml. of water and sulfuric acid was added dropwise until a clear solution was obtained. The pH at this point was 3.8. 10 grams of barium chloride were added with stirring and the solution was filtered. 24.2 grams of potassium fiuotitanate monohydrate were added and the solution was made up to 3 liters with water.

The aluminum pieces selected as cathodes for plating were 2 inch by 6 inch panels of 28 alloy, 2 inch by 6 inch panels of 2024 alloy, and a 3 inch by 3 inch die cast panel of 380 alloy. A carbon anode was used. 3.2 volts and 60 amperes per square foot were used. Plating was carried out at a bath temperature of F.

-At the makeup pH of 3.8 the electroplated zinc was not adherent on any of the test alloys. The pH was then dropped in 0.1 pH units from 3.8 to 0.3 by additions of sulfuric acid and each of the three alloys was plated at each stage. It was observed that throwing power was very good above a pH of 1.5 but that adhesion was poor above 1.9. It was also observed that adhesion was satisfactory from pH 0.5 to 1.0 and was excellent from 1.0 to 1.9, but there was some sacrifice of appearance and throwing power below 1.5. The plates from the satisfactory range were given a variety of adhesion tests including immersion in oil at 450 F. The zinc electroplate alone; the zinc electroplate with hard chrome; and the zinc electroplate with copper, nickel, chrome overplates were given standard corrosion tests. All test results were equal to or in excess of automobile and aviation specifications.

Example 3 A 3 liter bath was made up as follows: 320 grams of a hydrated zinc sulfate (70 percent anhydrous) and 24 grams of potassium fluotitanate were dissolved in 3 liters of water. Sulfuric acid was added dropwise until the pH was 1.7. Plating was carried out according to the procedure of Example 2.

The bath, as made up, contained 75 grams per liter anhydrous zinc sulfate and the plating procedure yielded a dirty looking plate of poor coverage. Zinc sulfate Was added to yield increased increments of anhydrous zinc sulfate of 25 grams per liter from a starting point of 75 grams per liter to saturation. The pH was corrected to 1.7 at each addition of zinc sulfate. 1

A satisfactory plate was first observed at a zinc sulfate concentration of grams per liter. Appearance and throwing power increased significantly to about 350 grams per liter and to some degree up to about 500 grains per liter. No additional advantage was observed in concentrations between 500 grams per liter and saturation. Again plates from the ranges of the invention were given the standard tests for adhesion and corrosion resistance. Successful results were obtained.

Example 4 A 3 liter bath was made up as follows: 900 grams of a hydrated zinc sulfate (70 percent anhydrous), one gram of potassium fluotitanate monohyd-rate, and 10 ml. of sulfuric acid were dissolved in water and brought to a volume of 3 liters. Plating was carried out according to the procedure of Example 2. The pH was 1.7.

The bath, as made up, contained 0.2 gram filuotitanate ion and the plating procedure yielded a plate of unsatisfactory coverage of zinc. Increments of 0.2 gram per liter fluotitanate ion were added from the starting point of 0.2 gram per liter to 12 grams per liter.

A satisfactory plate was first observed at a fluotitanate ion concentration of 0.6 gram per liter. There was significant improvement to a peak at 5.0 grams per liter fluotitanate and a gradual loss thereafter to about 10 grams per liter fluotitanate. At concentrations in excess of about 10 grams per liter fiuotitanate the throwing power of the bath was again considered unsatisfactory. Plates from the satisfactory ranges, those of the invention, successfully passed all standard adhesion and corrosion tests.

Example A 3 liter bath was made up as follows: 1000 grams of hydrated zinc sulfate (70 percent anhydrous), 25 grams of potassium fluozirconate, and 10.6 ml. of sulfuric acid were dissolved in water and brought to a volume of 3 liters. The pH was 1.65. Plating was carried out according to the procedure of Example 2 except that the temperature of the bath was raised in 5 F. increments from 80 F. to 150 F. and at each temperature stage current density on the cathode was varied from about amperes to about 3-00 amperes per square foot.

A temperature range of 100-140 F. was most suitable aud a temperature of 120 F. was best. A current density range of about -200 appeared most suitable and particularly about 60 amperes per square foot. The plated samples, in the suitable ranges indicated,'had perfect adhesion of the zinc electroplate. These plates took a successful hard chrome deposit directly on the zinc deposit when plated in a standard hard chrome tank. They also were plated with the usual copper, nickel, chrome decorative sequence to produce beautiful adherent plates.

I claim:

1. A plating bath for the electroplating of zinc on an aluminum basis metal said bath consisting essentially of an aqueous solution of zinc sulfate in a concentration of at least about 175 grams per liter and about 0.6-10 grams per liter of an ion selected from the class consisting of fluotitanate' and fluozirconate ions and said bath having a pH of about 0.5-1.9 and being substantially free of chloride ions and free titanium ions.

2. The bath according to claim 1 wherein the the fiuotitanate ion.

3. The bath according to claim 1 wherein the the fluozirconate ion.

' 4. A plating bath for the electroplating of zinc on an aluminum basis metal said bath consisting essentially of an aqueous solution of zinc sulfate in a concentration of about 175-500 grams per liter, about 1-15 grams per liter of a compound selected from the class consisting of alkali metal fiuotitanates and alkali metal fiuozirconates, said bath having a pH of about 1.5-1.9 and being substantially free of chloride ions and free titanium ions.

5. The bath according to claim 4 wherein said compound is an alkali metal iiuotitanate.

6. The bath according to claim 4 wherein said compound is an alkali metal fluozirconate.

ion is ion is 7. The bath according to claim 4 wherein said compound is potassium fiuotitanate.

8. The bath according to claim 4 wherein said compound is potassium fluozirconate.

9. A plating bath for the electroplating of zinc on an aluminum basis metal said bath consisting essentially of an aqueous solution of about 175-500 grams per liter zinc sulfate and about 8 grams per liter potassium fluotitanate, said solution having a pH of about 1.7 and being substantially free of chloride ions and free titanium ions.

10. A method of electroplating zinc on an aluminum basis metal comprising the step of passing a suitable current through a plating bath between an inert anode and said aluminum basis metal as a cathode; said bath being at a temperature of about -140 R, having a pH of about 0.5-1.9 and being substantially free of chloride ions and free titanium ions, and consisting essentially of an aqueous solution of zinc sulfate in a concentration of at least about 175 grams per liter and about 06-10 grams per liter of an ion selected from the class consisting of fluotitanate and fluozirconate ions.

11. The method according to claim 10 wherein said ion is fiuotitanate.

12. The method according to claim 10 wherein said ion is fiuozirconate.

13. The method according to claim 10 and comprising the added step of periodically adding to the bath to adjust the pH upward to a point below about 1.9 a base selected from the class consisting of zinc oxide, zinc hydroxide, and zinc carbonate.

14. A method of electroplating zinc on an aluminum basis metal comprising the step of passing a suitable current through a plating bath between a carbon anode and said aluminum basis metal as a cathode, said bath being at a temperature of about 100'-140 F., having a pH of about 1.5-1.9 and being substantially free of chloride ions and free titanium ions, and comprising essentially an aqueous solution of zinc sulfate in a concentration of about 175-500 grams per liter and about 1-15 grams per liter of a compound selected from the class consisting of alkali metal fiuotitanates and alkali metal fiuozirconates.

15. The method according to claim 14 wherein said compound is an alkali metal fluotitanate.

16. The method according to claim 14 wherein said compound is an alkali metal fiuozirconate.

17. The method according to claim 14 wherein said compound is potassium fluotitanate.

18. The method according to claim 14 wherein said compound is potassium fiuozirconate.

19. A method of electroplating zinc on an aluminum basis metal comprising the step of passing a suitable current through a plating bath between a carbon anode and said basis metal as a cathode; said bath being at a temperature of about F., having a pH of about 1.7 and being substantially free of chloride ions and free titanium ions, and comprising essentially an aqueous solution of about -500 grams per liter zinc sulfate and about 8 grams per liter of potassium fiuotitauate.

20. A method of electroplating zinc on an aluminum basis metal comprising a first step of passing a suitable current through a plating bath between an inert anode and said aluminum basis metal as a cathode; said bath eing at a temperature of about 100140 F., having a pH of about 0.5-1.9 and being substantially free of chloride ions and free titanium ions, and consisting essentially of an aqueous solution of zinc sulfate in a concentration of at least about 175 grams per liter and about 06-10 grams per liter of fiuotitanate ion; and further comprising the steps of periodically adding activated carbon to said bath, then adjusting the pH above about 4.8, then filtering, and then adjusting the pH to about 0.5-1.9.

21. A method of electroplating zinc on an aluminum basis metal comprising a first step of passing a suitable current through a plating bath between a carbon anode and said aluminum basis metal as a cathode; said bath being at a temperature of about 120 F., having a pH of about 1.5-l.9 and being substantially free of chloride ions and free titanium ions, and consisting essentially of about 175-500 grams per liter zinc sulfate and about l-15 grams of potassium fluotitanate; and further comprising the steps of periodically adding about grams per liter of activated carbon to said bath, then adjusting the pH to about 4.8, then filtering, and then adjusting the pH to about 1.5-1.9.

22. A method of preparing a plating bath for the electroplating of zinc on an aluminum basis metal comprising the steps of preparing an aqueous solution of zinc sulfate in a concentration of at least about 175 grams per liter, adding to the solution a soluble barium compound to yield about l-4 grams of barium ions per liter, filtering, adding to the solution about 1-15 grams of a member of the group consisting of soluble fiuotitanates and soluble fiuozirconates, and adjusting the pH of the solution to about 0.5-1.9.

23. The method according to claim 22 wherein said member is an alkali metal fluotitanate.

24. The method according to claim 22 wherein said member is an alkali metal fiuozirconate.

25. The method according to claim 22 wherein said member is potassium fiuotitanate.

26. A method of preparing a plating bath for the electroplating of zinc on an aluminum basis metal comprising the steps of preparing an aqueous solution of zinc sulfate in a concentration of about 175-500 grams per liter, adding about 1.5-6 grams of barium chloride per liter, filtering, adding about 1-15 grams per liter of potassium fiuotitanate, and adjusting the pH of the solution to about 1.51.9.

27. A method of preparing and maintaining a plating bath for the electroplating of zinc on an aluminum basis metal comprising the first set of steps of preparing an aqueous solution of zinc sulfate in a concentration of at least about 175 grams per liter, adding to the solution a soluble barium compound to yield about 1-4 grams of barium ions per liter, filtering, adding to the solution about 1-15 grams of a soluble fiuotitanate, and adjusting the pH of the solution to about 0.5-1.9; and further comprising the second set of steps of periodically adding activated carbon to said solution, adjusting the pH above about 4.8, filtering, and adjusting the pH to about 0.5-1.9.

28. The method according to claim 27 wherein said fluotitanate is an alkaline metal fiuotitanate.

29. The method according to claim 27 wherein said fluotitanate is potassium fluotitanate.

30. A method of preparing and maintaining a plating bath for the electroplating of zinc on an aluminum basis metal comprising the first set of steps of preparing an aqueous solution of zinc sulfate in a concentration of about 175-500 grams per liter, adding about 1.5-6 grams of barium chloride per liter, filtering, adding about l-15 grams of potassium fluotitanate, and adjusting the pH of the solution to about l.5-l.9; and further comprising the second set of steps of periodically adding about 10 grams per liter activated carbon, adjusting the pH to about 4.8, filtering, and adjusting the pH to about 1.5-1.9.

31. A method of preparing, plating from, and maintaining a plating bath for the electroplating of zinc on an aluminum basis metal comprising the first set of steps of preparing an aqueous solution of zinc sulfate in a concentration of about 175-500 grams per liter, adding about 1.5-6 grams of barium chloride per liter, filtering, adding about 1-15 grams per liter of potassium fiuotitanate, and adjusting the pH of the solution to about 1.5-l.9; comprising the second set of steps of passing a suitable current through said solution between a carbon anode and said basis metal as a cathode and at a temperature of about -140 F. and periodically adding to the solution to readjust the pH upward to a point below about 1.9 a base selected from the group consisting of zinc oxide, zinc hydroxide, and zinc carbonate; and further comprising the third set of steps of periodically adding about 10 grams per liter of activated carbon to said solution, adjusting the pH of said solution to about 4.8, filtering, and adjusting the pH to about 1.5-1.9.

References Cited in the file of this patent UNITED STATES PATENTS 2,300,693 Oswald Nov. 3, 1942 2,365,889 Tinsley Dec. 26, 1944 2,537,032 Chester et al. Jan. 9, 1951 2,543,545 Faust et al Feb. 27, 1951 2,824,058 Zimmerman Feb. 18, 1958 2,872,415 Schleyer et al. Feb. 3, 1959 2,952,590 Stareck et al Sept. 13, 1960 FOREIGN PATENTS 17,313 Australia Dec. 11, 1928 22,226 Great Britain Sept. 28, 1895 of 1894 1,211,108 France Oct. 1959 OTHER REFERENCES Smith: The Metal Industry, November 10, 1939, pages 415-417. 

1. A PLATING BATH FOR THE ELECTROPLATING OF ZINC ON AN ALUMINUM BASIS METAL SAID BATH CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OF ZNC SULFATE IN A CONCENTRATION OF AT LEAST ABOUT 175 GRAMS PER LITTER AND ABOUT 0.6-10 GRAMS PER LITER OF AN ION SELECTED FROM THE CLASS CONSISTING OF FLUOTITANATE AND FLUOZIRCONATE IONS AND SAID HAVING A PH OF ABOUT 0.5-1.9 AND BEING SUBSTANTIALLY FREE OF CHLORIDE IONS AND FREE TITANIUM IONS. 